In prior art magneto-optical (MO) disk drives, data is read as a clockwise or counter-clockwise polarization rotation imposed on a polarized laser light by the up or down orientations of magnetic domains within an area of stored data. The minimum area that the data can comprise is a function of the size of an optical spot formed by the polarized light. The information embedded in the polarization rotation requires an optical readout means. The optical readout means in the prior art includes a plurality of bulky and complex optical elements, some of which are located on a magneto-optical head. The optical elements can degrade the signal to noise ratio (SNR) of the information signal obtained from the polarization rotation.
Magnetic based disk drives are also well known in the art. In magnetic drives, magneto-resistive elements on magnetic heads are typically used. Recent advances in magnetic recording technology have provided magnetic heads that use giant magneto-resistive (GMR) technology; see, for example, “Giant Magnetoresistance: A Primer”, by Robert White, IEEE Transactions On Magnetics, Vol. 28, No. 5, Sep. 1992, incorporated herein by reference. GMR heads may be manufactured to be more sensitive to magnetic fields than conventional magneto resistive heads. GMR technology has also been incorporated with Spin-Valve structures that are well known in the art.
Magnetic storage drive technology is subject to the “super paramagnetic limit”, which simply states that when longitudinally oriented (in plane) magnetic domains are written In granular thin film metallic media, these domains will tend to demagnetize each other at some high flux reversal density and at high temperature.
Today the smallest mark written by a 220 KBPI PR-4 recording system, at say a 2 μm track pitch, is about 0.1 μm long along the track by 1.8 μm wide across the track. The north-to-south poles of the longitudinal domains are oriented along the track. Typically, there are at least four limitations encountered in prior art magnetic storage drive technology when trying to make the track pitch more narrow, including:
1) For tracking, a very high servo error rejection needs to be implemented, which requires a coarse and fine tracking actuator, for example, a crossover frequency of greater than 2 Khz is required for a track pitch of 1 μm.
2) The photolithographic tolerances for making a read/write head is limited to about 10% of the thickness of the head permalloy poles. If the poles are about 4 μm thick, the head tolerances are about 0.4 μm.
3) A typical prior art head causes side-erasure of data tracks of about 0.3 μm. This side erasure is desired for proper operation during the record/playback process so as to eliminate old information. Because, side-erasure scales with the head gap width, the gap width will be a function of a desired magnetic domain density.
4) The writing of the radial and circumferential position sensing patterns (servo writing) is not very accurate because of disk flutter and spindle bearing non-repeatable runout. One can typically expect a servo writing accuracy of about 0.2 μm with 0.8 mm thick 3.5 inch disks spinning on a ball bearing spindle.
What is needed therefore is an improvement that takes into account the limitations of the prior art magnetic and optical drive technologies.